In conventional Internet protocol (IP) phone systems, a session initiation protocol (SIP) has widely been used. The SIP is a communication protocol used to establish sessions between multiple mobile wireless devices, such as mobile phones.
Furthermore, as for the architecture of an IP core system for mobile phone systems, an IP multimedia subsystem (IMS) is defined in the 3rd generation partnership project (3GPP: the 3rd generation mobile communication system). The IMS is used as an IP core network architecture for voice communication based on the international mobile telecommunications (IMT)-2000 standard being a wireless communication network standard for 3G services, or for voice communication based on the long term evolution (LTE) standard for 3.9G services.
Moreover, a transition from the 3G services to the 3.9G services is currently in progress, and enhancements to wireless communication networks are also in progress. Therefore, a system called single radio voice call continuity (SRVCC) has been established that performs handover from an existing wireless communication network (for example, a 3G network) to a new wireless communication network (for example, a 3.9G network) to compensate for a narrow area of the new wireless communication network.
Specifically, when a voice call service based on the new wireless standard (for example, 3.9G) is to be provided, it is difficult to establish wireless communication networks that cover the whole country at the beginning of the service. Therefore, the service is started in a limited area in the early period and the area is gradually expanded.
Incidentally, because a wireless area corresponding to the new wireless standard is narrow in the early period, if a dual device that can cope with both of the new wireless standard and the existing wireless standard moves to the outside of the area of a new wireless standard network, handover to an existing wireless standard network is performed. Therefore, a voice service can be continued over a wide range, so that the narrow wireless area corresponding to the new wireless standard may be compensated for without reducing the convenience of the service.    Patent Document 1: Japanese Laid-open Patent Publication No. 2008-219436
However, in the conventional technologies, a technology is not taken into account, which centrally manages a call state of the dual device by using a simple structure with minimum modifications to the functions used in the existing wireless communication network.
Specifically, a case will be described below that an existing wireless standard service (for example, a 3G service) is adapted to an IMS network including multiple servers that perform processes using Internet protocol version 4 (IPv4) addresses. In this case, if a new wireless standard service (for example, a 3.9G service) is to be introduced, because IPv4 IP addresses are being depleted, it is expected to establish the new wireless standard service by an IP network using IPv6 addresses.
In this case, it is assumed that, when the IP version differs between the IPv4 IMS network and the IPv6 IMS network as described above, a dual device starts voice communication while being located in the area of the IPv6 IMS network and thereafter moves to the IPv4 IMS network, and the SRVCC is performed. In this case, because the IMS network controlling a call due to the voice communication is the IPv6 network, the dual device may continue to access the IPv6 IMS network via an address conversion system that converts IP versions even after the dual device has moved to the IPv4 IMS network.
In contrast, if a second call due to call waiting (CW) or the like is performed after the SRVCC, the second call from the 3G network is performed over the IPv4 IMS network. Therefore, the first call is managed in the IPv6 IMS network while the second call is managed in the IPv4 IMS network. Consequently, in some cases, it may become difficult to determine the call state, such as an incoming call (or a busy state).
Incidentally, if an application server (AS) in the IPv4 IMS network and an AS in the IPv6 IMS network cooperate with each other so that the ASs can mutually refer to the call state, it may become possible to centrally manage the call state. However, if a certain function is added to the server, such as the AS, in the IPv4 IMS network, communication failures or the like may occur with respect to existing users using the IPv4 IMS network.
Therefore, it is preferable to centrally manage the call state of a dual device by using a simple structure with minimum modifications to servers in the IPv4 IMS network.